With development of metallocene catalysts with excellent catalytic activity and selectivity, industrial-scale processes of preparing polyolefin by polymerizing olefin in the presence of a solid catalyst in a gas phase medium are widely used. In this gas-phase polymerization process, olefin may be polymerized by mechanically stirring a reaction bed (stirred bed reactor) or by continuously circulating a reaction gas to fluidize the reaction bed in a suspension state (fluidized bed reactor) in order to maintain a polymer bed through which the reaction gas flows.
The polyolefin prepared by the gas-phase polymerization process may include a large amount of residual monomers, and these monomers may cause explosion during a processing step of products and may be mixed with final products to deteriorate properties of the products or to produce defective products. Therefore, the monomers must be removed from the final products.
Previously, to remove residual monomers from polyolefin prepared by the gas-phase polymerization process, the polyolefin was purged with nitrogen gas as a purge gas to vaporize residual monomers, which were then separated from the polyolefin, as in Korean Patent Publication No. 2011-0084161.
When monomers having 4 or less carbon atoms are mainly used in the gas-phase polymerization process, residual monomers can be separated only by the step of purging with nitrogen gas as in the above-described patent. However, when alkenes having 5 or more carbon atoms are used as monomers, there is a limitation that alkenes having 5 or more carbon atoms cannot be removed only by the step of purging with nitrogen gas, because of high boiling point of the monomers.
Accordingly, there is a need to develop a method of purifying polyolefin, which is applicable to the gas phase polymerization process using alkene monomers with high carbon numbers and is used to remove residual monomers in a simpler manner.